There are a variety of optical measuring and testing applications in which light is incident upon a surface and then selected parameters of the reflected or transmitted light, such as intensity or polarization, are measured. Thus, it may be desired to measure surface roughness, thin film layer thickness, etch depth of various surface structures, or linewidth. Various common techniques include spectrometry, interferometry, polarimetry, ellipsometry, scatterometry, reflectometry and transmissometry. For example, one way to measure the dimensions (width or thickness) and shape of lines formed on a generally flat, reflective semiconductor wafer surface is to provide a test pattern of such lines formed as a high frequency grating in designated areas on the wafer surface interspersed among the integrated circuit patterns, then to illuminate the test pattern with monochromatic light from a laser source, and to detect and measure the resulting reflectance at various angles. From an analysis of the measured angle-dependent reflectance data, for example by comparison with reference data obtained from either a theoretical model or previous measurements done on calibration standards, the desired line parameter or parameters can be determined.
One type of optical instrument for carrying out this measurement is described in U.S. Pat. Nos. 4,710,642 (McNeil) and 5,164,790 (McNeil et al.) and in a paper by S.S.H. Naqvi et al. in the Journal of the Optical Society of America A, vol. 11, no. 9, pages 2485-2493, September 1994. FIG. 1 illustrates the basic configuration from the Naqvi et al. paper. The measurement instrument is described as an angle-scanning scatterometer in which a spot 10 on a sample 12 is illuminated by a laser beam 14 incident at an angle .theta..sub.i relative to the surface normal of the sample. Normally, the laser wavelength and incidence angle are fixed for a given measurement. Scattered light 16 is detected by a photodiode 18 over a range of scattering angles .theta..sub.s at a constant radius from the point of incidence 10 on the sample surface, as indicated by the circular detector path 20 in FIG. 1. For this purpose, the photodiode 18 may be mounted on a rotary stage (not shown) with its rotation axis passing through the illumination spot 10. In this instrument, the specularly reflected beam 22, and usually the first order diffraction beams, are removed from the data. The photodiode output may be amplified, sampled by an analog-to-digital converter, and analyzed in a microprocessor or computer 24. This apparatus, which directs a laser beam directly on the sample surface offers the advantage of being able to control the illumination area. However, the measurement is serial in nature, and therefore slow. The above-referenced '642 McNeil patent describes a variation in which a detector array, positioned in an arc extending partially around the sample, is used instead of a movable photodiode. Here, the entire range of scattering angles is detected at one time. Additional detector arrays are provided to measure the radiation of the incident beam when the sample is not in place and also to monitor the specularly reflected light to ensure proper orientation of the sample.
Another type of instrument employing angle-dependent intensity measurements is disclosed by Gold et al. in U.S. Pat. No. 4,999,014. FIG. 2 illustrates this configuration. There the measurements are used to determine the thickness of a thin film layer 30 on the surface of a sample substrate 32. A laser 34 generates a beam 36 that is reflected downward by a beamsplitter 38 towards the sample 32. A lens 40 having a large (at least 0.5) numerical aperture ("N.A.") focuses the beam to a spot 42 on the thin film layer 30. The N.A. of the lens 40 creates a spread of incidence angles from substantially normal to the thin film surface for the central (chief) ray to at least 30.degree. from normal for the extreme ray. A preferred embodiment has an 0.95 N.A. lens, giving a spread of greater than 70.degree.. The incident beam is reflected upwardly by the sample back through the lens 40, through the beamsplitter 38 and onto a photodetector array 44. Each discrete detecting element of the photodetector 44 corresponds to an incidence (and reflectance) angle at the sample surface. In a preferred embodiment there are two orthogonal linear arrays of detectors corresponding to respective S and P polarization components of the light. The intensity information obtained by the detector array 44 is used by a processor 46 to calculate the thin film thickness (and index of refraction) of the layer 30 on the sample 32. The apparatus provides parallel measurement, wherein light at many angles of incidence (and reflection) are intercepted by different elements of the detector array and detected simultaneously. In addition to providing a large angular spread for the incident and reflected light, the large N.A. lens 40 also focuses the light down to a very small spot 42 on the sample surface 30, typically on the order of 1 .mu.m diameter or less, so that only a very small area is illuminated and highly localized variations in the sample parameters can be measured. While such a small spot is highly useful for the thin film thickness measurements for which the instrument was invented, it would be completely unsuitable when line width or other parameters are to be determined from a periodic surface structure, like a test pattern. For such a measurement, a much larger illumination area, on the order of 10 .mu.m diameter, is required, like that provided by the instrument in FIG. 1.
Serial measurement over a 50.degree. to 60.degree. range of reflection angles, one degree at a time, is relatively inefficient, typically taking about a minute to complete, including the time needed to physically move the detector to each measurement position. Parallel measurement by a fixed detector array would be much faster, taking as little as a millisecond to complete. Unfortunately, the diffraction properties of light create a conflict between a high N.A. objective lens used to obtain a large range of incidence and reflection (or transmission) angles over which simultaneous measurements are taken and the large illumination area needed for measurements of a periodic test pattern on a wafer surface. The spot size of a light beam at the focus is proportional to .lambda./N.A., where .lambda. is the wavelength of the light and N.A. is the numerical aperture of the optical system. The proportionality constant is usually between 0.6 to 1.2, depending on the definition of beam size and other factors, such as the degree of coherence of the beam. It can be seen from this relationship between normal aperture and the beam size at the focus that the larger illumination areas required for periodic test patterns necessitates optics with a much smaller numerical aperture, or even direct laser beam illumination as in the instrument shown in FIG. 1, thereby reducing the range of angles over which a measurement is taken. Likewise, if large numerical aperture optics are used to obtain a larger range of incidence and reflection (or transmission) angles for simultaneous measurement, then a small illumination spot results. Illuminating a relatively large area, e.g., on the order of 10 .mu.m diameter, is important in those measurement applications where the effects of light interaction with periodic surface structures are needed, and also where it is desired that the measurement be an average over the illumination area so as to be relatively insensitive to localized variations in the sample properties.
Accordingly, it is an object of the present invention to provide angle-dependent optical measurements simultaneously over a wide range of incidence and reflection (or transmission) angles for an illumination area that is many times larger than the diffraction limit of the objective lens.